Mirror protective coating



Patented Aug. 5, 1947 MIRROR PROTECTIVE COATING James H. Smith, Vallejo,Calif.

No Drawing. Application June so, 1945?, Serial No. 602,651

(Granted under the act of March 3, 1883, as amended April 30, 1928.; 3700. G. 757) 3 Claims.

This invention relates to an improved mirror protective coating ormaterial for application to mirror backs.

The reflecting layer normally provided on mirrors is extremely thin andis easily damaged. It is likewise susceptible to peeling from its glassor other transparent support, particularly under adverse environmentalconditions. Accordingly, to protect the reflecting layer, and thereby toprolong mirror life, it is customary to provide the back surfaces of themirror with a protective coating, as of a varnish or paint.

The great majority of protective coatings for mirrors now known in theart give satisfactory results under normal conditions. A special problemhas arisen, however, in connection with the maintenance of mirrors usedaboard ship, and particularly those used in fire and engine rooms wherethe atmosphere is moist and hot. When placed in such locations, mirrorsprovided with any one or more of the protective coatings heretoforeknown in the art last but a short time, the reflecting layer oftenpeeling away from its base in a matter of a few weeks time.

The best protective coating known to have been developed to date forusage under these conditions is a paint having iron oxide as a majoringredient. However, even those mirrors which have been coated with theiron oxide paint do not last as long as it is desired they should.

It is therefore an object of this invention to provide an improvedprotective coating for use on mirror backs.

A more particular object is to provide such a coating which adequatelyprotects the reflecting material layer of mirrors which are used underadverse environmental conditions, as in a hot, moist atmosphere.

Other objects of this invention will become apparent from the followingdescription and appended claims.

I have found that the life of mirrors used aboard ship in fire andengine rooms can be greatly extended by providing the back thereof, thatis, the rearward, or open, side of the reflecting layer, with a coatinghaving chlorinated rubber as its principal ingredient. This coating isapplied in the form of a liquid which contains from about 10 to about 30per cent chlorinated rubber, either natural rubber, as guayule or theEast India type, or a synthetic rubber product, as neoprene or Buna-S,being satisfactory; from about 1 to about 10 per cent of a suitableplasticizing ingredient, as tricresyl phosphate or one of thechlorinated diphenyls; and from about 5 2 to about 25 per cent of boneblack or an equivalent carbonaceous material. The balance of thesolution normally consists of a solvent for the chlorinated rubberingredient, as xylol, carbon tetrachloride, butyl acetate, benzol, orthe like.

A preferred solution i one containingabout 15 to 20 per cent of therubber ingredient, about 2 to 5 per cent of the plasticizer, and about13 to 20 per cent bone black, with solvent making up the remainder.These ingredients may be added in any desiredv order, but the resultingmixture should be stirred until a smooth solution is obtained.

The above described solution is applied by brush, or otherwise, to theback of the mirror. It may be placed directly in contact with thereflecting layer, or an intervening layerof some other protectivematerial may be used, the rubber solution disclosed herein beingapplied, in turn, over any such intervening protective layer or layers.

Once the solution is in place, the solvent portion thereof is caused tobe evaporated, thereby leaving the back of the mirror provided with atough, resilient, and highly adherent coating which serves to protectthe underlying reflecting material layer under even the most extremeenvironmental conditions. When desired, this evaporation process can bespeeded up by placing the coated mirror in an oven maintained at asomewhat elevated temperature.

While the protective coating here disclosed is resilient, and therebyaids in protecting the mirror from damage by shock, it is alsosufiiciently brittle that it breaks cleanly along the mirrors cleavagelines when the mirror is cut. This quality permits the protectivecoating here disclosed to be applied to large mirror back areas when themirror is in the shop and can be treated under ideal conditions. Thecoated mirror can then be shaped as desired by cutting withoutdestroying its protective coating.

I have found that the above described combination of qualities can bestbe achieved by utilizing the solution whose composition has beendisclosed above.

Example To illustrate the manner in which this invention findsapplication, the following example is given. A coating solution wasprepared which consisted of 18 per cent chlorinated guayule; 2 per centtricresyl phosphate; 16 per cent bone black; and 64 per cent xylol. Thissolution was viscous, but was easily spread over the back surface of arelatively large mirror. In this case, the protective coating wasapplied directly over the silver reflecting layer, and no interveningprotective materials were utilized. The applied rubber material wasallowed to dry at room temperatures, and upon drying, it set to form atough, resilient coating which was firmly joined to the underlyingreflecting material layer. The coated mirror was then cut into severalsmaller mirrors by using a stylus to scratch the glass and then breakingthe glass along the scratch lines by pressure. The rubber back on themirror was found to have broken cleanly along the line of each cut, orbreak, in the glass, and in no place did the protective coating pullaway from the back of the mirror. The smaller mirrors so obtained werethen placed for test in the engine room of a ship. Periodic examinationthereafter showed that the life of the mirrors coated in this fashionwas at least twice that of mirrors which were protected in the bestfashion heretofore known to the art.

' The various percentages employed herein, and

I in the appended claims, refer to percentage by weight of the entiresolution or mixture under discussion.

While this invention is described in terms of particular ingredients,and ranges thereof, to be used,.it is obvious that many modificationsand variations in the nature and proportions of the ingredients may bemade without departing from the spirit and scope of the invention, andonly such limitations should be imposed as are indicated in the appendedclaims.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

I claim:

' and about 64 per cent xylol.

1. A composition for application to mirror backs to provide an improvedprotective coating thereon, consisting substantially of from about 15 toabout 20 per cent chlorinated rubber, from about 2 to about 5 per centof a plasticizing ingredient for said clorinated rubber, and from about13 to about 20 per cent bone black, the balance of the compositioncomprising a solvent for said chlorinated rubber.

2. A composition for application to mirror backs to provide an improvedprotective coating thereon, consisting substantially of about 18 percent chlorinated Guayule, about 2 per cent tricresyl phosphate, about 16per cent bone black,

3. As a. new article of manufacture, a mirror having firmly joined tothe rear portion thereof a protective coating formed by applying theretoa liquid composition comprising from about 15 to 20 per cent chlorinatedrubber, from about 2 to about 5 per cent of a plasticizing ingredientfor said chlorinated rubber, and from about 13 to about 20 per cent boneblack, the balance of the composition comprising a, solvent for saidchlorinated rubber, said composition being dried on said rear mirrorportion to form said coating.

JAMES H. SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS- Number Name Date 1,601,597 Peacock Sept. 28, 19261,750,583 Ellis Mar. 11, 1930 1,852,998 Ellis Apr. 5, 1932 2,044,176McCulloch June 16, 1936 2,091,714 7 Matthews Aug. 31, 1937 2,234,317Pfeiffer Mar. 11, 1941

